The past three decades has seen a tremendous increase in activity in the field of scanning probe microscopy (SPM). A sub-field of SPM called near-field scanning optical microscopy (NSOM) or scanning near-field optical microscopy (SNOM) has focused on techniques that surpass the diffraction limit for microscope resolution (˜0.5 μm, the wavelength of light) as stipulated by Abbe and Rayleigh; indeed, SNOM having a lateral resolution of 0.01 the wavelength of light has been demonstrated. The approach that has been used to achieve this extraordinary resolution is a technique named scattering scanning near-field optical microscopy—sSNOM or also referred to as apertureless near-field optical microscopy—aNSOM. This technique is essentially based on U.S. Pat. No. 4,947,034 (ref 1). sSNOM working in the infra-red range of the spectrum (5 μm to 20 μm) has demonstrated the ability to measure infra-red absorption spectra of molecules with spatial resolution approaching 10 nm (ref 2). However, molecular resolution cannot currently be achieved due to poor signal to noise ratio—which gets worse as the wavelength of the probing radiation is increased. Furthermore, working in the infra-red is cumbersome requiring expensive tunable lasers and components. An alternative approach to perform molecular spectroscopy is to detect tip enhanced Raman scattering (TERS) (ref 3). In this technique, a laser beam at frequency (op is focused on the gold or silver tip of a scanning probe microscope system (scanning tunneling microscopy (STM) or atomic force microscopy (AFM)). The polarization of the incident laser is arranged such that the electric field oscillation of the incident beam has a component along the axis of the SPM probe tip. The intense field enhancement produced under the tip is scattered by the vibrating molecules (which vibrate at their eigen frequencies ωm1, ωm2, ωm3 etc). A small fraction (10−7) of the incident power at ωP is converted into frequencies that are both up shifted (ωP+ωm1, ωP+ωm2, ωP+ωm3 etc.) anti-Stokes bands and down shifted (ωP−ωm1, ωP−ωm2, ωP−ωm3 etc) Stokes bands. In TERS microscopy/spectroscopy, the scattered radiation at ωP is suppressed using suitable filters while the up shifted or down shifted bands are transmitted and analyzed in a high resolution spectrometer to identify the vibrational modes of the molecule under the tip. TERS has demonstrated the capability to measure the vibrational frequencies of single molecules. However, the detected signals are extremely weak; in the best cases, typically 100 photons/second are detected from a single vibrational resonance and from molecules having very high Raman cross sections. Typically, the molecules have to be driven at or near an electronic resonance to enhance the signal. TERS is unable to measure the phase response of a molecular vibration.
Coherent anti-Stokes Raman and Stokes Raman Scattering techniques have been demonstrated with diffraction limited optics (best resolution is one half the wavelength of light); they have been successfully applied to microscopy and spectroscopy (ref 4). These techniques provide a much improved signal to noise ratio and are capable of measuring Raman spectra without the need for resonant electronic enhancement. Because the techniques rely on third order non-linear processes, they require femtosecond lasers with high peak powers for successful operation. In femtosecond stimulated Raman spectroscopy (FSRS), the entire Raman spectrum can be captured by a single pump-probe pulse pair (ref 5). In FSRS, the Stokes signal is amplified by stimulated emission and because there is a π/2 phase shift of the incoming stimulating beam as it passes through its focus, the Stokes signal constructively interferes with the stimulating beam and appears as a series of peaks on top of the stimulating background. The signal to noise ratio is much enhanced compared with spontaneous Raman detection. In femtosecond pump-probe experiments, phase information about the vibrating molecules can in principle be retrieved by recording the interference signal (as described above) versus frequency and time in conjunction with suitable simulation models. This is however an indirect measure of molecular phase (ref 6).